The present invention relates to a headlamp for automobiles which is intended to form a dipped beam.
This beam is characterised by a "cut-off", that is to say a directional limit above which no light rays are emitted. This cut-off generally consists of a horizontal half-plane to the left of the horizontal axis of the headlamp (for driving on the right-hand side of the road) and a half-plane which is slightly inclined upwards to the right of the said axis. This latter half-plane is raised by an "angle of upward inclination of the cut-off" which for a standard European beam is 15.degree..
The illumination produced by such a beam on a screen placed 25 meters in front of the headlamp is shown in FIG. 1, with its standard points and zones, the point H being the trace of the focal axis of the headlamp at the intersection of the vertical plane v'v and the horizontal plane h'h. The cut-off is defined by the trace Hh' of the left-hand horizontal plane lowered by 1% and by the trace Hc forming an angle .alpha. with respect to the trace Hh (here, as below, the description refers to driving on the right-hand side of the road. For driving on the left-hand side of the road it is sufficient to consider the Figures showing the screen or the headlamp reversed with respect to the axis v'v).
The zone III located above the cut-off is a zone of minimal illumination in order to avoid dazzle. The zone IV, on the other hand, is the zone of maximum illumination for which a strong intensity of the beam must be sought. Conventionally the cut-off is obtained by means of a screening cap which surrounds the lower part of the bulb or its filament and thus only allows the passage of the rays directed towards the top of the reflector associated with the bulb which after reflection will form the lower part of the beam. In order to obtain the required focusing, the filament of the bulb is arranged in the axis of the parabolic reflector, slightly in front of the focus thereof.
The disadvantage of this arrangement is the significant loss of luminous flux emitted by the filament because of the screening caused by the cap. Thus almost half the flux is emitted as pure loss. It will be understood that this loss is particularly critical for headlamps of small dimensions for which the reduced size of the reflector only permits recovery of sufficient luminous flux at the expense of increasing the power of the light source.
In order to avoid the use of a cap, a headlamp having the following structure has been proposed:
a reflector of which at least one sector is in the form of a paraboloid of revolution extending symmetrically on either side of the axis between two axial planes, one horizontal and the other forming with the latter an angle equal to the angle of upward inclination of the cut-off of the dipped beam,
bulb with an axial filament, this filament being on the one hand offset upwards in the radial direction with respect to the axis of the paraboloid and on the other hand centered in the axial direction on the focus of the paraboloid, and
a light-distributing glass placed in front of the reflector in which the zones which are homologous to those of the sector in the form of a paraboloid are smooth or slightly deviatory.
Such an arrangement of the elements of the headlamp is described notably in French Patent Specification No. A-1546698 in the name of the present applicants. It makes it possible to produce the cut-off because of its property of forming images all situated below the latter. However, the sector in the form of a paraboloid is very narrow (the angle of aperture .alpha. is generally 15.degree.), and in order to maintain acceptable efficiency it is necessary to recover the luminous flux corresponding to the rays not reflected by the sector in the form of a paraboloid.
For this the aforementioned document proposes placing two recuperator mirrors on either side of the sector in the form of a paraboloid, formed by two offset semi-paraboloids; the upper one focused on the rear end of the filament forms a conventional dipped beam and the lower one focused on the front end of the filament forms all its images below the cut-off.
Such a headlamp has two disadvantages: first of all the reflector has a discontinuous surface located at the junction of each recuperator mirror with the central sector: the paraboloids of the two adjacent surfaces, focused on different points, of necessity have either a different apex or different focal lengths and consequently have different profiles along the connecting plane. Therefore in this plane it is impossible to find a common connecting line and the transition of a recuperator mirror to the central sector of necessity represents a break. Because of this characteristic a reflector produced in accordance with this teaching is in practice imperfect in this transition zone, which manifests itself by an emission of light rays above the cut-off.
In the second place, and above all, the beam produced by the lower recuperator is spread over almost all of the zone situated below the cut-off: this widening of the beam goes against the desired aim in a dipped beam which is to obtain concentration in a central zone just below the cut-off (notably the standard zone IV).
It is for this reason that the said soluton has not been adopted in order to produce a dipped beam and that, in practice, the cut-off has always up to now been obtained by means of a screening cap.